Truncus arteriosus (also known as common aorticopulmonary trunk, truncus arteriosus communis, and singleoutlet heart) refers to a single large ventricular outflow tract arising from both right and left ventricles. This single large outflow tract, or truncus, gives rise to the coronary arteries, aorta, and pulmonary arteries.

In the original anatomic classification of Collett and Edwards (1949), four subtypes were described:

Type I demonstrates a single pulmonary trunk arising from the truncus, which then subdivides into right and left pulmonary arteries.

Type II demonstrates two pulmonary arteries arising directly from the posterior surface of the truncus.

Type III demonstrates two pulmonary arteries arising from the lateral aspects of the truncus.

Type IV demonstrates absent pulmonary arteries, but collateral arteries arising from the descending aorta supply the pulmonary vasculature.

Van Praagh and Van Praagh (1965) subsequently described a different classification system in which type A truncus is associated with a ventricular septal defect (VSD) and type B truncus is not associated with a VSD. Type A truncus is divided further into four subtypes, which closely resemble the classification of Collett and Edwards:

Type A1 demonstrates a partially separated pulmonary trunk.

Type A2 demonstrates two pulmonary arteries arising directly from the truncus.

Type A3 demonstrates a single pulmonary artery arising from the truncus together with further collaterals arising from the descending aorta.

Type A4 demonstrates significant arch anomalies in association with the truncus.

Additionally, The Society of Thoracic Surgeons has further modified the classification by combining Types A1 and A2 from the Van Praagh and Van Praagh classification system (Jacobs, 2000).

Almost 60% of cases of truncus are type I, based on the classification of Collett and Edwards, 35% are type II, 5% are type III, and a small minority are type IV (Bharati et al., 1974). In 65% of cases, the single truncus seems to arise predominantly from the right ventricle that may make differentiation from tetralogy of Fallot difficult. The truncus arises entirely from the right ventricle in 15% of cases, straddles the ventricular septum equally in a further 15% of cases, and arises mostly from the left ventricle in less than 5% of cases (Bharati et al., 1974). The single valve within the truncus is known as the truncal valve, and it is tricuspid in more than 65% of cases, although the number of leaflets may range from one to six (Bashore, 2007). Truncal valve dysplasia is very common, although significant truncal stenosis is rare, occurring in only 5% of cases (Bharati et al., 1974). Truncal valve regurgitation may be common, occurring in up to 40% of cases (Di Donato et al., 1985).

Associated cardiac malformations are common with truncus arteriosus; a VSD is present in the vast majority of cases. Right-sided aortic arch, atrial septal defect, abnormal origin of brachiocephalic vessels, aortic coarctation, atrioventricular canal defect, mitral atresia, and abnormal cardiac situs have all been described in association with truncus (Bharati et al., 1974). Approximately 50% of patients will have a patent ductus arteriosus in childhood, and approximately 20% will have an interrupted aortic arch (Bashore, 2007). Extracardiac abnormalities include situs inversus, heterotaxy syndrome (see Chapter 56), DiGeorge syndrome (see Chapter 139), and genitourinary anomalies (Collett and Edwards, 1949; Bharati et al., 1974).

Truncus arteriosus is a rare form of congenital heart disease, accounting for only 1.5% of all severe cardiac malformations seen during infancy (Fyler et al., 1980). Its incidence is estimated to be 1 in 10,000 livebirths and may be slightly more frequent in females (Hoffman and Kaplan, 2002). Truncus arteriosus likely accounts for only 7% to 10% of prenatally diagnosed conotruncal malformations (Sivanandam et al., 2006). The incidence of truncus arteriosus may be increased 12-fold in the presence of pregestational diabetes mellitus in the mother (Ferencz et al., 1990).

Prenatal diagnosis of truncus arteriosus is possible with obstetric sonography and targeted fetal echocardiography. Visualization of the outflow tracts should demonstrate a single large ventricular outflow tract, overriding a VSD (Romero et al., 1988) (Figures 54-1 and 54-2). The right ventricular outflow tract should be absent, and pulmonary arterial branches may be visible arising from the truncus or from the descending aorta. A distended coronary sinus may also be visible directly posterior to the left atrium (de Araujo et al., 1987). Diagnosis in utero may also be aided by the identification of additional anomalies, such as those involving the aortic arch, as well as by Doppler identification of truncalvalve regurgitation (Marasini et al., 1987). Truncus arteriosus may also lead to the development of congestive heart failure in utero, leading to the typical sonographic features of hydrops.

A transverse view through the upper fetal mediastinum, which normally contains the main pulmonary artery, ascending aorta, and superior vena cava, should demonstrate only two vessels in cases of truncus arteriosus (Yoo et al., 1997). The importance of visualization of the outflow tracts is evident froma series of six cases of prenatally diagnosed truncus arteriosus in which only two of the six cases were detected with the four-chamber cardiac view (Paladini et al., 1996). The remaining four cases were detected using a combination of four-chamber and outflow tract views.

In the hands of experienced fetal medicine and pediatric cardiology specialists, accurate prenatal diagnosis of the relationship of the arterial outflow tracts is possible. In a series of 113 consecutive fetuses with conotruncal malformations, including 8 cases of truncus arteriosus, the great arterial spatial relationship was correctly identified in 92% (Sivanandam et al., 2006). In an Australian population database study of prenatal detection of congenital heart defects, 18 of 27 cases (67%) of truncus arteriosus were successfully identified prenatally (Chew et al., 2007).